Technical Field
The present invention relates to a low-profile antenna device, and more particularly to a low-profile antenna device for a vehicle.
Background Information
At present, a variety of antenna devices are mounted on a vehicle. Such antenna devices include, for example, an AM/FM radio antenna that can receive an AM broadcast and an FM broadcast. A rod antenna is generally used for an AM/FM radio antenna. The rod antenna is configured with an element section in which an element is formed h a conductor having a spiral shape (helical element) covered with a cover, and a base plate on which the element section is attached.
When the rod antenna is attached to a vehicle body, a large part of the element section protrudes from the vehicle body, which may spoil the appearance and design of the vehicle body, and the element section may be damaged at the time of garaging, or washing the vehicle. The element section, which is exposed to the outside of the vehicle, may also be stolen.
To cope with the above problem, there has been proposed a low-profile antenna device whose height is made lower than that of a rod antenna, the element of which is housed in an antenna case so as to be protected from exposure to the outside of a vehicle, and the antenna case of which is configured to have a fukahire shape (shark fin shape) in consideration of the design of the entire vehicle carrying an antenna. Many of such low-profile antenna devices have a height lower than or equal to 70 mm and a longitudinal length of around 200 mm, in consideration of laws and regulations and the like.
However, such an antenna device has a problem that, due to an influence from a conductor loss of an antenna (a shortened element length) caused by a low-profile of 70 mm or less, radiation efficiency tends to be lowered, which causes sensitivity deterioration. For example, Japanese Patent Application Kokai Publication No. 2012-204996(Patent Document 1) discloses a low-profile antenna device, in which a coil is inserted between a top-load element and an amplifier circuit for the purpose of eliminating sensitivity deterioration. In Patent Document 1, the top-load element is disposed on an element holder that is provided to stand on a circuit board, and a coil is disposed on the element holder. Since performance degradation of the coil causes lowering in an antenna gain, a holding section is provided on the element holder in order to dispose the coil around the middle between the top-load element and the circuit board, so that a conductor that lowers a Q value of the coil is not present around the coil. A predetermined feeding terminal is used or complicated forming is required for connection between the board and the coil.